Signaling through the T cell receptor (TCR) complex is controlled by the invariant CD3delta, epsilon, gamma and zeta chains. Each CD3 chain possesses one or more copies of the immunoreceptor-based tyrosine activation motif (ITAM), which acts as a binding site for intracellular factors. The individual functions of different CD3 chains and their respective ITAMs remain to be established. CD3delta interacts preferentially with TCRalpha, suggesting a unique role in signaling through the alpha/beta-TCR as opposed to other TCR isoforms lacking the TCRalpha subunit. Consistent with this hypothesis, CD3delta is essential for alpha/betaTCR-mediated positive selection and activation of the ERK signaling pathway thought to control this process. In contrast, CD3delta is not normally essential for the preTCR-mediated DN to DP transition, but can become so under certain experimental conditions. Interestingly, the roles of CD3delta in alpha/betaTCR- and preTCR-mediated processes appear to map to different functional domains. Here we propose to define the mechanistic basis for these different roles by addressing the following specific questions. 1. a) Does CD3delta-dependent preTCR-signaling require a functional CD3delta ITAM, and if so is there a specific requirement for the CD3delta ITAM? b) Are the signaling pathways initiated by the preTCR and gamma/deltaTCR isoforms altered in the absence of CD3delta, and in particular is CD3delta required for specific activation of the ERK pathway? c) What is the functional basis for the role of CD3delta in the gamma/deltaTCR-mediated DN to DP transition? 2. a) Are the roles of CD3delta in alpha/betaTCR assembly and positive selection distinct, and if so what regions of CD3delta do they map to? b) Is the apparent requirement for the CD3delta EC.B region in positive selection specific to this process and to the ERK signaling pathway, and what is the structural basis for this requirement? 3. a) Are the homologous CD3delta and gamma ITAMs collectively essential for particular signaling pathways and developmental processes? b) Is the CD3delta ITAM individually sufficient to supply all ITAM-mediated functions essential for development?